This invention relates to ionization current detectors for internal combustion engines having a plurality of ignition coils and detecting ionization current flowing through the ignition coils during combustion of a mixed gas.
FIG. 3 shows a circuit diagram of a conventional detector, in which reference numeral 1 designates an ignition coil having a primary side 1a and a secondary side 1b and arranged on each of cylinders of an internal combustion engine; 2, a power transistor connected to the primary side 1a for cutting off the primary current; 3, a spark plug connected to the secondary side 1b for firing a mixed gas of a not shown internal combustion engine by applying a high ignition voltage; 4, a capacitor connected to the positive pole of the secondary side 1b; 5, a resistor inserted between the capacitor 4 and a ground for converting ionization current to a voltage; and 6, a diode connected to the resistor 5 in parallel therewith.
Reference numeral 7 designates a Zener diode inserted between the secondary side 1b and a coil power supply 10; 8, an AC coupler circuit for extracting only an AC component out of the voltage obtained at the resistor 5; 9, a comparator circuit for comparing the voltage obtained at the AC coupler circuit 9 with a predetermined reference level; 11, an ionization current detector circuit consisting of the components designated by reference numerals 4 through 9. As many power transistors 2 and ionization current detector circuits 11 as ignition coils 1 are provided. Reference numeral 12 designates an OR circuit for receiving outputs of the respective ionization current detector circuits 11 and ORing these outputs; and 13, an output terminal for outputting a combustion pulse from the OR circuit 12 when the ionization current has been detected.
In the above organization, when each power transistor 2 turns off at an ignition timing of the internal combustion engine and the primary current flowing through the primary side 1a is then cut off, a negative high ignition voltage is generated at the secondary side 1b. As a result, a spark occurs across the electrodes of each spark plug 3, firing the mixed gas in the internal combustion engine. At this point, ionization takes place in association with combustion of the mixed gas, producing ions.
Here, the electrodes of each spark plug 3 act as electrodes for detecting the ionization current after the spark. The ionization current flows by the movement of electrons caused by the positively biased voltage of the capacitor 4. The generation of the ionization current in turn generates a voltage across each resistor 5. The AC component of this voltage is extracted by each AC coupler circuit 8, and the extracted AC component is compared with the reference level at each comparator circuit 9, whereby a combustion pulse is generated from each comparator circuit 9. The combustion pulses from these comparator circuit 9 are ORed at the OR circuit 12, producing a combustion pulse from the output terminal 13. Combustion of the mixed gas is verified by detection of such combustion pulse.
The thus organized conventional detector requires that as many ionization current detector circuit 11 as ignition coils be provided, which has made the organization complicated and expensive.